The invention generally relates to the field of protection technology for single-phase or polyphase low-voltage power supply systems. In one embodiment, it can be applicable to the circuitry configuration of an undervoltage release. Such a release can include, for example, an electromagnet for tripping a switching device, when the voltage of the power e supply system falls below a predetermined magnitude, and a driver circuit feeding the electromagnet, with the driver circuit having a rectifier circuit, an auxiliary voltage source, a pulse transmitter for producing a pulsed holding current for the electromagnet, and a threshold value circuit, which is provided with an undervoltage detector, for interrupting the holding current.
In one known undervoltage release, the rectifier circuit is used to obtain a direct current, the auxiliary voltage source which is arranged downstream from the rectifier circuit is used for supplying voltage to the electronics, and the pulse transmitter is used for producing a holding current which, provided the power supply system voltage is sufficiently high, is essentially independent of this power supply system voltage. In this case, the pulse transmitter has an operational amplifier and an electronic switch which is controlled by the operational amplifier and is connected in series with a resistor in the holding circuit of the electromagnet, which is connected to a freewheeling diode. Furthermore, the pulse transmitter contains a feedback path to allow the voltage which is dropped across the resistor to act on the input of the operational amplifier (U.S. Pat. No. 4,788,621 A, DE 195 07 936 A1). It is also known for an undervoltage release such as this to be combined with an overcurrent release, with the overcurrent detector acting optoelectronically on a further electronic switch which is arranged in parallel with the electromagnet and the freewheeling diode and can short-circuit them (U.S. Pat. No. 4,890,184).
In the known undervoltage releases, the freewheeling diode which is arranged in parallel with the electromagnet is used itself to hold the electromagnet when the voltage briefly falls below a minimum level or in the event of brief interruptions. The tripping delay that occurs in this case is more than 100 ms. It is known for the tripping delay to be increased by using a capacitor and by a parallel arrangement comprising a series circuit of a resistor and a further electronic switch for the resistor that is connected in series with the electromagnet (DE 195 07 936 A1).
However, sometimes it is also desirable to ensure that the electromagnet trips in as short a time as possible (less than 100 ms) when the voltage briefly falls below a minimum voltage or in the event of brief voltage interruptions.
An object of an embodiment of the invention is to provide a circuit arrangement which is suitable.
In order to achieve an object, an embodiment of the invention provides for a further electronic switch which can be driven as a function of the output signal from the undervoltage detector and which is arranged between the electromagnet and the first-mentioned electronic switch, to be arranged in the self-holding circuit which is formed from the electromagnet and the freewheeling diode. Further, a timer may be provided for driving the further electronic switch, one pole of which timer is coupled to the auxiliary voltage source via a diode circuit.
The use of a timer for driving the second electronic switch is advantageous in comparison to the possible use of an optocoupler to drive it. The further electronic switch can expediently also be short-circuited via a link in order to make it possible to operate the undervoltage release with the xe2x80x9cnormalxe2x80x9d tripping delay, as well.
The refinement of the circuit arrangement provided according to an embodiment of the invention allows an additional electronic switch to be activated by mechanically releasing a short-circuiting link, with this additional electronic switch being located in the same circuit as the freewheeling diode and thus being able to interrupt the self-holding circuit of the electromagnet with a delay of less than 100 ms.
It is known per se from EP 0 203 366 A1 for two switching elements which are optionally in the form of electronic switches, to be provided for a control apparatus for electromagnetic switching devices, which switching elements are located in the circuit of an operating coil. One of these switching elements allows a freewheeling diode to be rendered ineffective in order, when required, to cause the current in the operating coil to decay more quickly. In this case, and in contrast to the invention, the primary factor is not measured monitoring of a power supply system voltage, but the protection of the switching device by influencing its own magnet drive. The known control apparatus therefore cannot be used as an undervoltage release. In addition to this, there are no details, corresponding to the invention, relating to the connection of a timer for controlling the switching element located in the self-holding circuit of the operating coil.